A resonant power converter is a power-electronics conversion apparatus based on a soft switching technology. With emergence of a high-frequency switch component, the resonant power converter develops towards high frequency, integration, and modularization. Soft switching is a switching process using the soft switching technology. In an ideal soft switching process, a current or voltage is first decreased to zero, and then the current or voltage is gradually increased to a rated value, so that a switching loss is approximately zero. Therefore, the power converter can have high frequency. Common soft switching includes: ZVS (zero-voltage switching), ZCS (zero-current switching), ZCT (zero-current transition), ZCT (zero-voltage transition), and the like.
An LLC resonant circuit (that is, including two inductors L and one capacitor C) is a common high-efficiency DC (direct current)/DC converter structure. To improve efficiency of an LLC resonant power converter, the LLC resonant power converter usually needs to be controlled to work in a soft switching status, so as to reduce a switching loss, thereby improving power density of the LLC resonant power converter. A direct current characteristic curve of the LLC resonant power converter is shown in FIG. 1, and Q in FIG. 1 is a quality factor of the LLC resonant power converter. The LLC resonant power converter may work at a resonant frequency point, in a ZVS area, or in a ZCS area. The LLC resonant power converter has best efficiency when working at the resonant frequency point. However, during actual working, the LLC resonant power converter may work in different areas because of a change in a parameter such as load or a voltage. To ensure security of the LLC resonant power converter, in all load ranges, the LLC resonant power converter needs to work at the resonant frequency point or in a first area in FIG. 1, and usually does not work in a second area, and cannot work in a third area. This is because if the LLC resonant power converter works in the second area, the LLC resonant power converter may enter the third area from the second area when the load increases, and monotonousness of a voltage gain changes (that is, originally, under specific load, the voltage gain decreases as frequency increases, and after the LLC resonant power converter enters the third area, under specific load, the voltage gain increases as frequency increases). Consequently, there is a breakdown risk. Therefore, the LLC resonant power converter needs to determine, by determining a resonant status of the LLC resonant power converter, whether the LLC resonant power converter works at the resonant frequency point, in the first area, in the second area, or in the third area, so as to adjust switching frequency of a switching transistor in the resonant power converter according to the resonant status.
Most existing resonant power converters determine resonant statuses of the resonant power converters according to a phase difference between a primary current that enters a resonant network and a square wave voltage applied to the resonant network. However, because a resonant power converter that performs sampling based on a secondary current cannot obtain the primary current of the resonant network, the resonant power converter that performs sampling based on a secondary current cannot determine a resonant status of the resonant power converter in an existing manner.